Cellulose synthesis

Shedletzky, E., Shmuel, M., Trainin, T., Kalman, S., and Delmer, D. (1992) Cell wall structure in cells adapted to growth on the cellulose-synthesis inhibitor 2,6-dichlorobenzonitrile. Plant Physiol. 100 120-130. 

The synthesis of cellulose by A. xylinum from various polyalcohols14 is accompanied by the formation of carbon dioxide, formic acid, nonvolatile acids, ketoses and sometimes ethanol. The much greater variety of substrates suitable for cellulose synthesis, as compared with the small number for dextran or levan, may account for the widespread natural occurrence of cellulose. 

The FP cellulose per unit (ml) volume and enzyme yield per unit (g) cellulose or substrate obtained on wheat straw, wood, and CTMP in SSF were higher than those obtained in LSF on wheat straw and wood (Tables I, II, and III). And wheat straw proved to be a better substrate than wood for cellulose production in SSF. This could be attributed to the polysaccharides (cellulose and hemicelluloses) of wheat straw being more readily available for the organism s growth and cellulose synthesis than those of wood. The hemicelluloses and cellulose were presumably not as available in wood, because of its high lignin content and high cellulose crystallinity, as in wheat straw. 

It seems likely that the enzyme complexes for hemicelluloses, pectins and cellulose are constructed, at least in part, on the endoplasmic reticulum and then transferred to the Golgi apparatus, where they are modified and sorted so that they can be segregated within the compartments of the Golgi cisternae (30,31). The complex for cellulose synthesis is not normally active within the Golgi apparatus and it is transported to active sites at the plasma membrane (1). The hemicelluloses and pectins are formed within vesicles and cisternae of the Golgi apparatus and the vesicles are transported to the plasma membrane, where fusion occurs and the polysaccharides are packed into the wall (1). It is not known whether particular polysaccharides such as the xylans of the hemicellulose and the arabinogalactans of the pectins are transported in separate vesicles or together in one vesicle. Nor is it known if the complex for cellulose synthesis is transported by vesicles which carry hemicellulose and pectin polysaccharides. 

Though cellulose is one of the most important biopolymers, it has not yet been possible to completely elucidate its biosynthetic pathway, or establish exactly the cell organellae involved in its synthesis. However, during the last decade, the freeze fracture technique has been applied to investigate cell wall formation, and this has produced much information on the site where cellulose synthesis occurs. It is now generally accepted that both terminal and rosette complexes are responsible for cellulose synthesis (21). Our results (19,22) support that view. In a TEM-autoradiographic investiga-... 

Our electron microscopy observations have revealed some of the roles of cell organellae involved in biosynthesis of cell wall components (i) the plasma membrane is the site of cellulose synthesis. This supports the proposal that terminal and rosette complexes at the plasma membrane are responsible for cellulose synthesis, (ii) The Golgi-bodies and small circular vesicles derived from the r-ER s are involved in the biosynthesis and/or transport of the hemicelluloses. Our investigations, however, could not distinguish between what type of cell organellae contained what kind of hemicelluloses, and how these polymers were processed in the organellae. 

PF had been proposed as the terminal complex (23) and associated pores were reported on the outer membrane EF (24). Due to their proximity to the site of cellulose ribbon extrusion from the cell surface, these structures were assumed to be responsible for cellulose synthesis. A model was advanced in which cellulose synthase was localized on the outer membrane, which invoked adhesion sites between the outer and plasma membranes as a mechanism to explain the transfer of uridine-diphosphoryl-glucose (UDPG) from the cytoplasm to the cellulose synthases (25,26). However, when the outer and plasma membranes of Acetobacter were isolated separately by density-gradient centrifugation, the cellulose synthase activity was localized only in the plasma membrane fraction (27). Therefore, the linear structures observed on the Acetobacter outer membrane, while they may be associated in some manner with cellulose biosynthesis, are probably not the cellulose synthase terminal complexes. Since no ultrastructural evidence for adhesion sites between the outer and plasma membranes has been presented, a thorough investigation of the mechanism of / (1-4) glucan chain translocation from the cytoplasmic membrane to the outer membrane in Acetobacter xylinvm is now in order. 

Figure 9. This model shows three left-handed helical submicrofibrils (SM) 1, 2 and 3 which emerged from the cell wall at their termini. It was not clear how the submicrofibrils first associated with other submicrofibrils but once associated they were spun together. This model assumed that cellulose synthesis provided the mechanical force that simultaneously extended and left-hand rotated the submicrofibrils, which in turn drove the secondary formation of three submicrofibrils into a left-hand helical microfibril (M).

Previous work has shown that the presence of Calcofluor or Tinopal could dramatically increase A. xylinum cellulose synthesis. This observation was the basis for the hypothesis that cellulose polymerization can be uncoupled from a slower sequential crystallization step (2-5). We believe the hypothesis is not consistent with our observations. At the very least, the presence of an ordered and crystal-like submicrofibril produced in the presence of 0.25 mM Tinopal would relegate Tinopal s or Calcofluor s effects to an event occurring after the initial cellulose polymerization-crystallization step or steps. 

The UDP-glucose used for cellulose synthesis is generated from sucrose produced during photosynthesis, by the reaction catalyzed by sucrose synthase (named for the reverse reaction) ... 

In one proposed model, cellulose synthase spans the plasma membrane and uses cytosolic UDP-glucose as the precursor for extracellular cellulose synthesis. In another, a membrane-bound form of sucrose synthase forms a complex with cellulose synthase, feeding UDP-glucose from sucrose directly into cell wall synthesis (Fig. 20-32). 

Certain bacteria (Acetobacter, Agrobacteria, Rhi-zobia, and Sarcina) and many simple eukaryotes also carry out cellulose synthesis, apparently by a mecha-... 

Cellulose synthesis takes place in terminal complexes (rosettes) in the plasma membrane. Each cellulose chain begins as a sitosterol dextrin formed inside the cell. It then flips to the outside, where the oligosaccharide portion is transferred to cellulose synthase in the rosette and is then extended. Each rosette produces 36 separate cellulose chains simultaneously and in parallel. The chains crystallize into one of the microfibrils that form the cell wall. 

Peng, L., Kawagoe, Y, Hogan, P., Delmer, D. (2002) Sitosterol-/3-glucoside as primer for cellulose synthesis in plants. Science 295, 147-150. 

Recent evidence for the lipid-oligosaccharide intermediate in cellulose synthesis. 

However, an enzyme that uses GDP-D-glucose as a substrate in order to synthesize a cellulose-like product has been reported to be present in developing cotton fibers during the period when primary-wall cellulose synthesis normally occurs, but is absent from older fibers undergoing active deposition of secondary-wall cellulose.303... 

The situation is complicated by the fact that the efficiency of cellulose synthesis in the cell-free system is low, and the same enzyme preparation catalyzes the incorporation of D-glucosyl groups from UDP-Glc into alkali-soluble (1 - 2)-/3-D-glucans.370 A similar process was reported to occur with a membrane preparation of Rhizobium meliloti,371... 

It has also been shown that mung beans, peas, and other plants contain a pyrophosphorylase which forms GDP-D-glucose from a-D-glucose 1-P and GTP. Based on the data obtained with enzymic plant preparations, we proposed the following mechanism for cellulose synthesis ... 

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